12 V Wire Gauge vs. Current and Length

Overview

Use this reference to select a copper American Wire Gauge (AWG) for a 12 V DC circuit when the design voltage drop is limited to 3% (0.36 V). The length in the table is the total circuit length: from the source to the load and back to the source. It is not the one-way distance.

This is a voltage-drop selection guide, not a code ampacity table. Conductor insulation, installation method, ambient temperature, terminations, and local electrical rules can require a different conductor or overcurrent-protection rating.

How to Use the Table

1. Measure the complete current path from the source to the load and back.
2. Choose the row for that total length and the column for the circuit current.
3. The intersecting value is the AWG size used by the source reference.

For example, a 15 ft total circuit carrying 15 A corresponds to 12 AWG. A 15 ft one-way run has a 30 ft total circuit length, so use the 30 ft row instead.

12 V Copper Wire Gauge Selection (3% Voltage Drop)

1 ft = 0.3048 m.

12 V Cable Table-Derived Maximum-Current Charts

These charts are calculated from the selection table above; they are not digitized copies of the original graphic. For each total circuit length and AWG, a plotted value is the highest current column in that row that specifies that AWG. If an AWG does not occur in a row, its point is blank rather than zero. The feet and metres charts show the same derived values.

Voltage-Drop Formula

For a two-conductor circuit, use the total circuit length directly:

$V_{\text{drop}} = I \times R_{1000} \times \frac{L_{\text{total}}}{1000}$

Where $I$ is current in A, $R_{1000}$ is the conductor DC resistance in $\Omega/1000\,\text{ft}$, and $L_{\text{total}}$ is the complete circuit length in ft. The resistance value must be in ohms, not milliohms.

Voltage Drop Calculator

The default resistance is 1.588 $\Omega/1000\,\text{ft}$ for 12 AWG solid copper at 20 °C. With the default 15 ft total circuit and 15 A load, the calculated drop is about 0.357 V (about 3.0%). Use manufacturer data for the actual conductor, temperature, and construction.

Important Limits

• Protect the conductor with correctly selected overcurrent protection. A fuse or circuit breaker cannot be chosen from AWG alone; use the applicable electrical code and the conductor manufacturer’s installation data.
• Resistance changes with conductor temperature and construction. Use the actual conductor’s published DC resistance when voltage drop is critical.
• If the chassis is used as a return path, include the resistance of the complete return path rather than assuming that only the supply conductor matters.
• This reference is for copper conductors in a 12 V system with a 3% voltage-drop design target. It does not replace code-required ampacity, derating, or termination checks.

References

• Original source — Engineering ToolBox
• Virginia Department of Energy — copper-wire resistance table (20 °C)